The present invention concerns a laser spark plug, an internal combustion engine having such a laser spark plug, and a method of ascertaining the operating condition of such a laser spark plug.
In laser ignition systems based on laser spark plugs of the general kind set forth, the ignition sparks are produced by focusing an intensive laser light pulse which lasts for only a few nanoseconds on the ignition location in the combustion chamber of the internal combustion engine. The laser light can be produced, for example, by a pump laser and an ignition laser (for example a laser crystal) connected downstream of the pump laser. A semiconductor laser which emits light over a plurality of milliseconds and which charges up the ignition laser is used as the pump laser. The ignition laser then delivers a laser light pulse which is in the order of nanoseconds in length and which is introduced into the combustion chamber of the internal combustion engine by way of the combustion chamber window.
Production of the laser light can suffer degradation over the operating time of the internal combustion engine, and the combustion chamber window through which the laser light pulse is introduced into the combustion chamber can suffer from transmission losses due to deposits at the surface towards the combustion chamber.
In total, the above issues can result in a considerable weakening in the ignition spark or (as a consequence) worsening of combustion in the internal combustion engine.
DE 10 2009 000 911 A1 discloses an ignition spark plug having at least one optical sensor which is integrated into the laser spark plug and which serves to monitor the energy content of the pump radiation for the laser crystal. However, that specification does not show detection of the ignition energy or light intensity which is actually of interest, being produced by the ignition laser or the laser crystal which is integrated into the laser spark plug. JP 2012-189044 A shows that a part of the light delivered by an ignition laser in the direction of the pump laser feeding the ignition laser is detected. In that case, light from the ignition laser that is delivered by the ignition laser in a direction away from the combustion chamber is detected. That detection also cannot provide any information as to how high the level of light intensity of the laser light is, which is delivered in the direction of the combustion chamber, in order to trigger an ignition spark.